Combined voltage, current control loops simplify solar apps

The rapid expansion of constant-current/constant-voltage (CC-CV) applications, especially in LED lighting and high capacity battery and supercapacitor chargers challenges power supply designers to keep pace with the increasingly complicated interplay of current and voltage control loops. A switch-mode converter designed specifically for CC-CV offers a clear advantage, especially when the supply has limited power, or its power is allocated among several competing loads.

Consider, for instance, the challenge of charging a supercapacitor in a minimum amount of time from a power-limited supply. To maintain constant input power, the controlled charging current must decrease as the output (supercapacitor) voltage increases. The LT3796 solves the problem of power limited or constant current/constant voltage regulation by seamlessly combining a current regulation loop and two voltage regulation loops to control an external N-channel power switch. The inherent wired-OR behavior of its three transconductance error amplifiers summed into the compensation pin, VC, ensures that the correct loop (that is, the one closest to regulation) dominates.

The additional, standalone current sense amplifier can be configured for any number of functions, including input current limit and input voltage regulation.

The LT3796ís wide VIN range (6V to 100V) and rail-to-rail (0V to 100V) output current monitoring and regulation allow it to be used in a wide variety of applications from solar battery chargers to high power LED lighting systems. The fixed switching frequency, current-mode architecture results in stable operation over a wide range of supply and output voltages. The LT3796 incorporates a high side current sense, enabling its use in boost, buck, buck-boost or SEPIC and flyback topologies.

Figure 1: A 34W LED driver with robust output short-circuit protectionClick on image to enlarge

@Semiman_#1: True. If you wanted to achieve temperature-invariant MPPT with this chip, it looks like you'd have to detach Vs and CSN from INTVCC, and re-attach the pins to an external temperature-sensing diode. You'd need to tweak the resistor values accordingly. I haven't shopped around, but it would seem reasonable for solar panel makers to include a temperature-sensing diode in the center of the panel; the panel's MPPT should track the diode's open-circuit voltage fairly well. If that's not the case, then you can always glue a diode to the panel and drive it with a current source (resistor to INTVCC).

It's a somewhat simplistic view of maximum power point tracking and would apply to one panel, at one temperature. It would not achieve anywhere near 100% utilization in the real world as the maximum power point voltage will vary considerably with temperature ... which is impacted by solar irradiance as well.

This chip has multiple features tied together and designers of UPS,inverters also can benefit. If this chip could be combined with a digital technology with a simple user interface to program various parameters of currents and voltages then it will be more easy to incorporate into many systems.